1. Field of The Invention
This invention relates to composite articles comprising metal oxide superconducting elements, and to a method of making such composite articles.
2. Description of The Related Art
A primary deficiency of metal oxide high temperature superconductor (MOHTSC) materials developed to date, particularly in large scale applications such as power transmission lines, generators, superconducting magnets, and motors, is that such oxide materials are, as ceramics, extremely brittle and highly sensitive to flaws. Any point stresses or defects in these materials cause local stress concentrations and provide a mode of mechanical failure which is at odds with the mechanical reliability necessary in such applications.
As a result of such intrinsic brittleness and susceptibility to point stresses and structural defects, MOHTSC materials are ill suited to use in applications such as motor coils and magnets, in which elements constructed of MOHTSC materials must withstand considerable stress, under complex modes of operation, without failure. As an example, it has been estimated that strains of several tenths of one percent will have to be accommodated in superconducting alternating current electric machines, as reported in Keim, T. A., "Extended Abstracts High Temperature Superconductors I," 1988, page 147. Based on elastic constants which have been reported (Leadbetter, H. M., et al, J. Mater. Res., 1987, Vol. 2, page 786; Leadbetter, H. M., J. Met., 1988, Vol. 40, No. 1, page 24) which have been reported for the so-called "123" MOHTSC materials (of the general formula YBa.sub.2 Cu.sub.3 O.sub.x, where x is from about 6 to about 7.1), superconducting wires with at least 50 ksi tensile strength would be required for large generator applications. In other applications such as transformers, motors, and small generators, and even in small-scale applications such as SQUID magnetometers and digital electronic components based on Josephson Junction Devices, physical properties requirements for the MOHTSC materials are less severe, but strength and mechanical reliability remain important considerations constraining the use of MOHTSC materials.
A further applications-related problem facing the implementation of MOHTSC materials is that environmental conditions which deplete oxygen from the MOHTSC material, e.g, high temperature exposure, can significantly adversely affect the superconducting properties (e.g., residual resistivity) of these oxide materials. For example, high temperature exposure can deplete MOHTSC materials of oxygen, with consequent adverse affect on the superconducting properties (e.g., residual resistivity) of the material.
Encasement of the brittle MOHTSC element, e.g., filaments or wires, in a matrix material is a potential approach to avoiding local stress concentrations which can result in failure of the element. The encasement medium, however, must be able to protect the superconductor from moisture and oxygen loss and must be a conductor that will not oxidize or deleteriously interact with the MOHTSC material. Further, any suitable matrix materials must be applicable to the MOHTSC element under conditions which do not cause chemical reactions with the MOHTSC material or otherwise deleteriously affect its composition and/or properties. In this respect, it is to be noted that the process conditions incident to the formation or deposition of the matrix material must likewise have no deleterious affect on the composition or performance properties of the MOHTSC material.
U.S. Pat. No. 4,845,308 issued Jul. 4, 1989 to E. A. Womack, Jr., et al describes a superconducting electrical conductor comprising an elongated porous substrate of bonded fibers, e.g., of aluminosilicate, with a coating of a 123 ceramic oxide superconductor. Such substrate is encased within an inner tube which is supplied with cryogen or other fluid for cooling the superconducting material. An outer tube surrounds the inner tube and defines a space which is evacuated for thermally insulating the inner tube from the ambient environment, with an electrical and heat insulator being provided around the outer tube. Adjacent lengths of the conductor are connected by a multiplicity of rigid rods each coated with superconducting material and held by a central plate-shaped perforated frame. The opposite ends of the rods are thrust into exposed ends of the porous substrate of the adjacent conductors. In this construction, the superconducting material is deposited onto and into the porous substrate of bonded fibers by methods such as chemical vapor deposition, evaporation deposition from a liquid or sol-gel solution, or by deposition from a precursor laden fluid.
U.S. Pat. No. 4,860,431 issued Aug. 29, 1989 to W. G. Marancik, et al describes the manufacture of multifilamentary intermetallic superconductors comprising Nb.sub.3 Sn. Plural copper tubes are filled with an alloy of tin to form copper-tin wires which are cabled around a core niobium wire. The resulting strands are provided in the copper tube, or a copper foil or finely wound copper wire and drawn to produce multifilament wire. Heat treatment then is employed to effect diffusion of tin and form the product intermetallic superconductor at the surface of the niobium filaments. The aluminum is said to appreciably increase the tensile strength of the tin and facilitate the processing of the composite material. The patent notes at column 4, lines 25-27 that the superconducting properties are not deteriorated by the addition of aluminum.
U.S. Pat. No. 4,857,675 to W. G. Marancik, et al discloses a forced flow type superconducting cable-in-conduit which is formed by cabling multifilamentary superconducting subcables around a first tube alternately with stainless steel wire or cable, and forming a second tube around the resulting composite. The double tube composite yielded by this procedure then is flattened to form a cable in a jacket. The superconductor in such structure may be Nb.sub.3 Sn or V.sub.3 Ga.
An approach to making wires out of superconductive material which has been proposed (see U.S. Pat. No. 4,079,187 to Fillunger et al; U.S. Pat. No. 4,377,032 to Benz; and U.S. Pat. No. 4,489,219 to Sunaga et al) involves embedding filaments of a non-MOHTSC superconductive material in an electrically conductive matrix such as copper or copper alloy. U.S. Pat. No. 4,529,837 to Borden discloses encasing non-MOHTSC superconductor filaments in a copper cladding, with the resulting wire bent to shape to form so-called Rutherford-type superconductor cable.
In connection with the foregoing, it is to be appreciated that the superconductor materials described in the above-discussed references, such as Nb.sub.3 Sn, V.sub.3 Ga, Nb.sub.3 Ge, etc., are not MOHTSC materials, and do not have the severe moisture and oxygen sensitivity problems which characterize the MOHTSC materials.
It is an object of the present invention to provide a superconducting composite article comprising MOHTSC material, wherein the MOHTSC material is protected from moisture and oxygen loss in use, so that the material and electrical properties of the MOHTSC material are maintained despite exposure to environmental conditions which would otherwise adversely affect same.
It is another object of the present invention to provide a superconducting composite article comprising an MOHTSC element which can be fabricated without adverse affect on the material and electrical properties of the MOHTSC constituent thereof.
It is a still further object of the present invention to provide a superconducting composite article comprising small diameter MOHTSC filaments whose mechanical properties and environmental stability are far superior to those of the filament per se.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.